This invention relates to digital video recording apparatus, such as a digital video tape recorder and more particularly, to a digital VTR intended for consumer use and operable in an after-recording mode to record a digital audio signal at a later time after a digital video signal has been recorded or alternatively, to record a digital video signal at a later time after the digital audio signal has been recorded.
In digital VTRs developed for consumer use, it is typical to record video and audio signals in digital form in different areas of the same track. For example, when recording sound and movement of a human subject, a video frame interval is recorded in a number of tracks and the sound uttered by the subject (referred to generally as the audio signal that is associated with the video signal) is recorded in those very same tracks, but at different locations. When recording in the NTSC standard, a frame interval of digitized video signals is recorded in ten tracks and the digitized audio signals associated therewith are recorded in the very same tracks. Of course, when the digitized audio signals represent sounds uttered by the subject, the recording of digitized video and audio signals in common tracks results in a video picture having proper lip synchronization, or movement between the subject's lips and the sounds emitted therefrom.
The recording of video and audio signals in separate portions of common tracks facilitates dubbing, voice overlay and other so-called special effect techniques. Hence, digital audio signals may be recorded at a later time, yet be associated with the previously recorded digital video signals. Conversely, video signals may be dubbed by editing digital video information onto a record medium which contains previously recorded audio information. The subsequent recording of digital audio signals that are intended to be associated with previously recorded digital video signals and the subsequent recording of digital video signals that are intended to be associated with previously recorded digital audio signals is referred to as after-recording and is a general description of re-recording audio or video signals after the basic recording of video and audio information is completed.
In a typical after-recording operation, the after-recorded digital audio (or video) signals are recorded in different tracks than the previously recorded digital video (or audio) signals associated therewith. These different tracks typically are located downstream (or delayed) from the originally recorded signals. When these signals are reproduced, a time difference is introduced between the reproduced digital video and audio signals which is manifested as a loss of synchronism therebetween. This is particularly noticeable and undesirable when the after-recorded signals are, for example, dubbed digital audio signals. In that event, when the reproduced digital video and audio signals are displayed as a video picture, there is a loss of synchronism between the movement of the subject's lips and the sound emitted therefrom. Stated otherwise, the operation of a digital VTR in its after-recording mode generally is associated with a loss of "lip sync."
The foregoing can best be appreciated by referring to FIGS. 1A-1C and 2A-2C of the accompanying drawings. FIG. 1A is a schematic representation of record tracks on a digital video tape in which digital video and digital audio signals are recorded. For the purpose of discussion, the basic information unit illustrated in FIG. 1A is a video frame interval; and the digital video signals included in this interval are recorded in ten tracks when using the NTSC format, and in twelve tracks when using the PAL format. The digital video signals are recorded in a major portion of each of these ten tracks; and the digital audio signal associated with that digital video signal, such as the audio signal that may be picked up simultaneously with the imaging of the video signal, also is recorded in these same ten tracks but, as shown in FIG. IA, in a smaller upper portion of each track. Thus, a unit of digital video signals is recorded in a ten-track segment and an associated unit of digital audio signals also is recorded in this ten-track segment.
FIG. 1B illustrates a timed relationship between the encoding, recording, reproducing and decoding of a digital video signal relative to the movement of the tape shown in FIG. 1A. A frame of digital video signals is supplied for recording during a period x.sub.1 and is encoded (for example, it is "shuffled") for recording in the ten-track segment during time interval x.sub.2. The encoding of the digital video signal is effected by digital processing circuitry in a matter known to those of ordinary skill in the art, and this processing circuitry exhibits an inherent time delay t.sub.1. This time delay t.sub.1 is seen to be equivalent the movement of 15 tracks of the video tape. Hence, this time delay t.sub.1 is referred to as a 15 track delay. From FIGS. 1A and 1B, is seen that a given point in a frame interval of a digital video signal is recorded in a track that is delayed by 25 tracks from the time that such point in the video signal is first supplied.
When the digital video signal is reproduced at a later time, it is played back during time interval x.sub.2 and then decoded (or "deshuffled") by digital processing circuitry normally used for this purpose. This decoding operation exhibits an inherent time delay t.sub.2 which, typically, may be approximately equal to the inherent time delay t.sub.1 exhibited by the encoder. Thereafter, the decoded digital video signal is recovered as an output video signal during time interval x.sub.3. It is seen from FIG. 1B that a time delay on the order of 25 tracks is present between the reproduction and the recovery of a given time point of a frame interval.
FIG. 1C illustrates the timing relationship in recording and reproducing an associated digital audio signal. A unit of audio signals (assumed to correspond to a frame interval) is supplied for recording during the interval x.sub.1 ; and this audio signal is encoded, such as for error correction, by digital audio processing circuitry of a type known to those of ordinary skill in the art. Such digital audio processing circuitry exhibits an inherent time delay t.sub.11 wherein t.sub.11 &lt;t.sub.1. As a numerical example, this inherent delay t.sub.11 is equal to a one track delay.
So that the digital audio signal may be recorded in the same tracks (but at separate portions) as the digital video signal associated therewith, the encoded audio signal is delayed by an amount t.sub.12 (ideally, t.sub.12 =t.sub.1 -t.sub.11) thereby bringing the digital audio signal into time synchronism with the encoded digital video signal. The digital audio signal then is recorded during time interval x.sub.2 in the same tracks as the digital video signal. Hence, the associated digital and audio signals are supplied to the recording heads in synchronism.
During a playback operation, the unit of digital audio signals is reproduced during the time interval x.sub.2, as shown in FIG. 1C, and then decoded (or error-corrected). This decoding operation exhibits an inherent time delay t.sub.21 (t.sub.21 &lt;t.sub.2); and to assure that the recovered digital audio signals are in synchronism with the associated digital video signals, the decoded audio signals are delayed by a time delay t.sub.22 (ideally, t.sub.22 =t.sub.2 -t.sub.21. Thus, by delaying the reproduced, decoded digital audio signals, a given time point in the unit of recovered audio signals is in proper synchronism with a corresponding time point in the recovered video signals. Consequently, during normal recording and reproduction, the video and audio signals are in proper synchronism and correct "lip sync" is present in the video picture recovered therefrom. That is, there is essentially no time difference between the reproduced digital and audio signals.
FIGS. 2A-2C illustrate how proper synchronism between previously recorded video signals and after-recorded audio signals is lost because of a substantial time difference that is present when the digital video and after-recorded digital audio signals are reproduced. FIG. 2A is a schematic representation of record tracks in which a frame of previously recorded digital video signals is reproduced from a 10-track segment during a playback period x.sub.2. In the after-recording mode, although digital audio signals also may be reproduced from this same 10-track segment, such digital audio signals are ignored and, thus, in the interest of simplification, a unit of digital audio signals is not illustrated as being recorded in this 10-track segment.
As occurred during a normal record/playback operation, the reproduced digital video signals are decoded by the aforementioned digital processing circuitry which, as discussed above, exhibits an inherent time delay t.sub.2. The decoded video signal then is recovered as the output video signal during time interval x.sub.3. The playback operation schematically represented in FIG. 2B is seen to be substantially identical to the playback operation schematically illustrated in FIG. 1B. Thus, a delay of about 25 tracks is present between the reproduction and the recovery (or output) of a given time point of a frame interval of the digital video signal.
Now, in an after-recording operation, a unit of the audio signal that is associated with this reproduced video signal is supplied for recording substantially immediately after the video signal is recovered. This timing relationship is schematically illustrated in FIG. 2C, wherein the audio signal is supplied during the interval x.sub.4 which is delayed by only 10 tracks (i.e. one frame interval) from the beginning of the frame interval of the recovered digital video signal with which this audio signal is associated. As before, this audio signal is encoded by digital audio processing circuitry exhibiting the inherent time delay t.sub.11, for example, a 1-track delay, and the encoded digital audio signal then is delayed by the amount t.sub.12 before it is recorded.
It is seen that, when an audio signal is supplied for recording in the after-recording mode, the inherent delay of the digital audio processing circuitry coupled with the additional delay normally imparted to the digital audio signal during a recording operation results in a time delay on the order of about 60 tracks from the location of a given time point in the digital video signal and the location of a corresponding time point in the after-recorded digital audio signal.
During a subsequent playback operation, the digital video signal is reproduced during time interval x.sub.2 and, 60 tracks later, the associated, after-recorded digital audio signal is reproduced during a time interval x.sub.5. As before, the digital video signal is decoded by digital processing circuity exhibiting the inherent time delay t.sub.2 and then is recovered as an output video signal during the time interval x.sub.3. The reproduced digital audio signal, on the other hand, is decoded by digital audio processing circuitry exhibiting the inherent time delay t.sub.21, and the decoded audio signal then is delayed by the time delay t.sub.22 such that the audio signal is recovered during the time interval x.sub.6. A comparison of FIGS. 2B and 2C indicates that, when the audio signal is recorded in the after-recording mode, the fact that the audio signal is recorded in tracks delayed from those in which the video signal is recorded, coupled with the additional time delay normally added to the reproduced audio signal for the purpose of assuring synchronism between the recovered video and audio signals results in a substantial time difference between the recovered digital video signal and the associated digital audio signal. This time difference is about 60 tracks, or 6 frames, which is approximately 0.2 seconds. Consequently, when a video picture is displayed from after-recorded signals, the video and audio signals are not in proper synchronism and "lip sync" is noticeably lost.